1. Field of the Invention
The present invention relates to an electrode having high capacity and good reversibility, and a secondary battery comprising i) a positive electrode comprising an organosulfur compound which is capable of reversible formation of S--S bond upon oxidation and a metallic compound selected from transition metals, and a current collector containing copper metal; ii) a polymer electrolyte having lithium salt; and iii) a negative electrode made of lithium metal, lithium alloy or lithium intercalation compounds.
2. Description of Prior Art
Batteries have a wide spectrum of applications as key component, of modern portable electronic devices. Especially, secondary batteries are essential to the development of hand-carrying devices such as cellular telecommunication tools and notebook computers. A series of development of nickel-cadmium, nickel-metal hydride, and lithium ion types have provided advantages in reducing the size and weight of secondary batteries. However, the rapid advancement of electronic technology and the widespread use of mobile devices has been continuously demanding a next generation battery which has higher capacity than existing systems.
The secondary battery using organosulfur compound as a positive electrode material has been disclosed in U.S. Pat. No. 4,833,048. In this patent, an S--S bond of organic disulfide compound consisting of positive electrode is cleaved by electrolytic reduction to form organic thiolate and organic disulfide is reformed by electrolytic oxidation of organic thiolate. Especially, in case of two or more thiolate groups present in a molecule, polymeric form of organic disulfide is formed. The redox couple of organic disulfide and organic thiolate accounts for theoretical energy density of 350 to 800 Wh/kg in combination with metal negative electrode. A rechargeable metal-sulfur battery described in the invention provides practically higher energy density of 150 Wh/kg than conventional secondary battery.
To increase the practical capacity of organic disulfide electrode, U.S. Pat. No. 5,324,599 suggested the addition of .pi. electron conjugated conductive polymer like polyaniline to cathodic composition containing organic disulfide. According to the report of same inventors disclosed in Nature, 373, 598(1995), the electron transfer of organic disulfide was catalytically accelerated in the presence of polyaniline. Accordingly, the composite electrode from organic disulfide and polyaniline mixed together in molecular level shows the enhanced energy density in excess of 600 Wh/kg when coupled with lithium metal as negative electrode. However, in order to maintain the high energy density, the cell required high charging voltage up to 4.75 V, which is too high to ensure the electrochemical stability of cell components such as polymeric electrolyte and other organic parts. Lower charging potential which is practically required results in the decline of energy density of the cell.
In order to increase the cycle life of organic disulfide electrode, approaches to immobilize organic disulfide have been made since diffusive loss of soluble form of organic disulfide, such as mercaptan or thiolate, eventually results in the decrease of capacity over the repeated cycle of charge and discharge. Addition of metal such as copper, or silver to bind organic disulfide species was diclosed in U.S. Pat. 5,665,492. Addition of copper ion to organic disulfide and use of the resulted complex was described in Eur. Pat. No. 799,264, A2. Improvement of cycle life was also suggested in U.S. Pat. No. 5,516,598, when metal salt of broad range of multivalent metallic complex of organic disulfide was used. In these disclosures, the role of metal as a coordinating center of sulfur containing ligand were suggested to improve the cycle life of organic disulfide electrode, but functions of metal such as redox reaction and activation of sulfur containing compound were not described. Consequently, above mentioned approaches provide only limited level of energy densities which is at maximum the sole capability of organic disulfide.
Accordingly, the improvement for enhancing capacity and extended cycle life has yet to be realized to the secondary battery employing organosulfur compound as positive electrode material.